@c Copyright (C) 1996-2022 The Octave Project Developers
@c
@c This file is part of Octave.
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@c Octave is free software: you can redistribute it and/or modify it
@c under the terms of the GNU General Public License as published by
@c the Free Software Foundation, either version 3 of the License, or
@c (at your option) any later version.
@c
@c Octave is distributed in the hope that it will be useful, but
@c WITHOUT ANY WARRANTY; without even the implied warranty of
@c MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
@c GNU General Public License for more details.
@c
@c You should have received a copy of the GNU General Public License
@c along with Octave; see the file COPYING.  If not, see
@c <https://www.gnu.org/licenses/>.

@node Input and Output
@chapter Input and Output

Octave supports several ways of reading and writing data to or from the
prompt or a file.  The simplest functions for data Input and Output
(I/O) are easy to use, but only provide limited control of how
data is processed.  For more control, a set of functions modeled
after the C standard library are also provided by Octave.

@menu
* Basic Input and Output::
* C-Style I/O Functions::
@end menu

@node Basic Input and Output
@section Basic Input and Output

@c We could use a two-line introduction here...

@menu
* Terminal Output::
* Terminal Input::
* Simple File I/O::
@end menu

@node Terminal Output
@subsection Terminal Output

Since Octave normally prints the value of an expression as soon as it
has been evaluated, the simplest of all I/O functions is a simple
expression.  For example, the following expression will display the
value of @samp{pi}

@example
@group
pi
     @print{} ans =  3.1416
@end group
@end example

This works well as long as it is acceptable to have the name of the
variable (or @samp{ans}) printed along with the value.  To print the
value of a variable without printing its name, use the function
@code{disp}.

The @code{format} command offers some control over the way Octave prints
values with @code{disp} and through the normal echoing mechanism.

@DOCSTRING(disp)

@DOCSTRING(list_in_columns)

@DOCSTRING(terminal_size)

@DOCSTRING(format)

@menu
* Paging Screen Output::
@end menu

@node Paging Screen Output
@subsubsection Paging Screen Output

When running interactively, Octave normally sends all output directly to the
Command Window.  However, when using the CLI version of Octave this can create
a problem because large volumes of data will stream by before you can read
them.  In such cases, it is better to use a paging program such as @code{less}
or @code{more} which displays just one screenful at a time.  With @code{less}
(and some versions of @code{more}) you can also scan forward and backward, and
search for specific items.  The pager is enabled by the command @code{more on}.

Normally, no output is displayed by the pager until just before Octave is ready
to print the top level prompt, or read from the standard input (for example, by
using the @code{fscanf} or @code{scanf} functions).  This means that there may
be some delay before any output appears on your screen if you have asked Octave
to perform a significant amount of work with a single command statement.  The
function @code{fflush} may be used to force output to be sent to the pager (or
any other stream) immediately.

You can select the program to run as the pager with the @code{PAGER} function,
and configure the pager itself with the @code{PAGER_FLAGS} function.

@DOCSTRING(more)

@DOCSTRING(PAGER)

@DOCSTRING(PAGER_FLAGS)

@DOCSTRING(page_screen_output)

@DOCSTRING(page_output_immediately)

@DOCSTRING(fflush)

@c FIXME: maybe this would be a good place to describe the following message:
@c
@c warning: connection to external pager (pid = 9334) lost --
@c warning: pending computations and output may be lost
@c warning: broken pipe

@node Terminal Input
@subsection Terminal Input

Octave has three functions that make it easy to prompt users for
input.  The @code{input} and @code{menu} functions are normally
used for managing an interactive dialog with a user, and the
@code{keyboard} function is normally used for doing simple debugging.

@DOCSTRING(input)

@DOCSTRING(menu)

@DOCSTRING(yes_or_no)

For @code{input}, the normal command line history and editing functions
are available at the prompt.

Octave also has a function that makes it possible to get a single
character from the keyboard without requiring the user to type a
carriage return.

@DOCSTRING(kbhit)

@node Simple File I/O
@subsection Simple File I/O

@cindex saving data
@cindex loading data
The @code{save} and @code{load} commands allow data to be written to and
read from disk files in various formats.  The default format of files
written by the @code{save} command can be controlled using the functions
@code{save_default_options} and @code{save_precision}.

As an example the following code creates a 3-by-3 matrix and saves it
to the file @samp{myfile.mat}.

@example
@group
A = [ 1:3; 4:6; 7:9 ];
save myfile.mat A
@end group
@end example

Once one or more variables have been saved to a file, they can be
read into memory using the @code{load} command.

@example
@group
load myfile.mat
A
     @print{} A =
     @print{}
     @print{}    1   2   3
     @print{}    4   5   6
     @print{}    7   8   9
@end group
@end example

@DOCSTRING(save)

There are three functions that modify the behavior of @code{save}.

@DOCSTRING(save_default_options)

@DOCSTRING(save_precision)

@DOCSTRING(save_header_format_string)

@DOCSTRING(load)

@DOCSTRING(fileread)

@DOCSTRING(native_float_format)

It is possible to write data to a file in a similar way to the
@code{disp} function for writing data to the screen.  The @code{fdisp}
works just like @code{disp} except its first argument is a file pointer
as created by @code{fopen}.  As an example, the following code writes
to data @samp{myfile.txt}.

@example
@group
fid = fopen ("myfile.txt", "w");
fdisp (fid, "3/8 is ");
fdisp (fid, 3/8);
fclose (fid);
@end group
@end example

@noindent
@xref{Opening and Closing Files}, for details on how to use @code{fopen}
and @code{fclose}.

@DOCSTRING(fdisp)

Octave can also read and write matrices text files such as comma
separated lists.

@DOCSTRING(dlmwrite)

@DOCSTRING(dlmread)

@DOCSTRING(csvwrite)

@DOCSTRING(csvread)

Formatted data from can be read from, or written to, text files as well.

@DOCSTRING(textread)

@DOCSTRING(textscan)

The @code{importdata} function has the ability to work with a wide
variety of data.

@DOCSTRING(importdata)

After importing, the data may need to be transformed before further analysis.
The @code{rescale} function can shift and normalize a data set to a specified
range.

@DOCSTRING(rescale)

@menu
* Saving Data on Unexpected Exits::
@end menu

@node Saving Data on Unexpected Exits
@subsubsection Saving Data on Unexpected Exits

If Octave for some reason exits unexpectedly it will by default save the
variables available in the workspace to a file in the current directory.
By default this file is named @samp{octave-workspace} and can be loaded
into memory with the @code{load} command.  While the default behavior
most often is reasonable it can be changed through the following
functions.

@DOCSTRING(crash_dumps_octave_core)

@DOCSTRING(sighup_dumps_octave_core)

@DOCSTRING(sigquit_dumps_octave_core)

@DOCSTRING(sigterm_dumps_octave_core)

@DOCSTRING(octave_core_file_options)

@DOCSTRING(octave_core_file_limit)

@DOCSTRING(octave_core_file_name)

@node C-Style I/O Functions
@section C-Style I/O Functions

Octave's C-style input and output functions provide most of the
functionality of the C programming language's standard I/O library.  The
argument lists for some of the input functions are slightly different,
however, because Octave has no way of passing arguments by reference.

In the following, @var{file} refers to a filename and @code{fid} refers
to an integer file number, as returned by @code{fopen}.

There are three files that are always available.  Although these files
can be accessed using their corresponding numeric file ids, you should
always use the symbolic names given in the table below, since it will
make your programs easier to understand.

@DOCSTRING(stdin)

@DOCSTRING(stdout)

@DOCSTRING(stderr)

@menu
* Opening and Closing Files::
* Simple Output::
* Line-Oriented Input::
* Formatted Output::
* Output Conversion for Matrices::
* Output Conversion Syntax::
* Table of Output Conversions::
* Integer Conversions::
* Floating-Point Conversions::
* Other Output Conversions::
* Formatted Input::
* Input Conversion Syntax::
* Table of Input Conversions::
* Numeric Input Conversions::
* String Input Conversions::
* Binary I/O::
* Temporary Files::
* EOF and Errors::
* File Positioning::
@end menu

@node Opening and Closing Files
@subsection Opening and Closing Files

When reading data from a file it must be opened for reading first, and
likewise when writing to a file.  The @code{fopen} function returns a
pointer to an open file that is ready to be read or written.  Once all
data has been read from or written to the opened file it should be closed.
The @code{fclose} function does this.  The following code illustrates
the basic pattern for writing to a file, but a very similar pattern is
used when reading a file.

@example
@group
filename = "myfile.txt";
fid = fopen (filename, "w");
# Do the actual I/O here@dots{}
fclose (fid);
@end group
@end example

@DOCSTRING(fopen)

@DOCSTRING(fclose)

@DOCSTRING(is_valid_file_id)

@node Simple Output
@subsection Simple Output

Once a file has been opened for writing a string can be written to the
file using the @code{fputs} function.  The following example shows
how to write the string @samp{Free Software is needed for Free Science}
to the file @samp{free.txt}.

@example
@group
filename = "free.txt";
fid = fopen (filename, "w");
fputs (fid, "Free Software is needed for Free Science");
fclose (fid);
@end group
@end example

@DOCSTRING(fputs)

A function much similar to @code{fputs} is available for writing data
to the screen.  The @code{puts} function works just like @code{fputs}
except it doesn't take a file pointer as its input.

@DOCSTRING(puts)

@node Line-Oriented Input
@subsection Line-Oriented Input

To read from a file it must be opened for reading using @code{fopen}.
Then a line can be read from the file using @code{fgetl} as the following
code illustrates

@example
@group
fid = fopen ("free.txt");
txt = fgetl (fid)
     @print{} Free Software is needed for Free Science
fclose (fid);
@end group
@end example

@noindent
This of course assumes that the file @samp{free.txt} exists and contains
the line @samp{Free Software is needed for Free Science}.

@DOCSTRING(fgetl)

@DOCSTRING(fgets)

@DOCSTRING(fskipl)

@node Formatted Output
@subsection Formatted Output

This section describes how to call @code{printf} and related functions.

The following functions are available for formatted output.  They are
modeled after the C language functions of the same name, but they
interpret the format template differently in order to improve the
performance of printing vector and matrix values.

Implementation Note: For compatibility with @sc{matlab}, escape sequences in
the template string (e.g., @qcode{"@backslashchar{}n"} => newline) are
expanded even when the template string is defined with single quotes.

@DOCSTRING(printf)

@DOCSTRING(fprintf)

@DOCSTRING(sprintf)

The @code{printf} function can be used to print any number of arguments.
The template string argument you supply in a call provides
information not only about the number of additional arguments, but also
about their types and what style should be used for printing them.

Ordinary characters in the template string are simply written to the
output stream as-is, while @dfn{conversion specifications} introduced by
a @samp{%} character in the template cause subsequent arguments to be
formatted and written to the output stream.  For example,
@cindex conversion specifications (@code{printf})

@example
@group
pct = 37;
filename = "foo.txt";
printf ("Processed %d%% of '%s'.\nPlease be patient.\n",
        pct, filename);
@end group
@end example

@noindent
produces output like

@example
@group
Processed 37% of 'foo.txt'.
Please be patient.
@end group
@end example

This example shows the use of the @samp{%d} conversion to specify that a
scalar argument should be printed in decimal notation, the @samp{%s}
conversion to specify printing of a string argument, and the @samp{%%}
conversion to print a literal @samp{%} character.

There are also conversions for printing an integer argument as an
unsigned value in octal, decimal, or hexadecimal radix (@samp{%o},
@samp{%u}, or @samp{%x}, respectively); or as a character value
(@samp{%c}).

Floating-point numbers can be printed in normal, fixed-point notation
using the @samp{%f} conversion or in exponential notation using the
@samp{%e} conversion.  The @samp{%g} conversion uses either @samp{%e}
or @samp{%f} format, depending on what is more appropriate for the
magnitude of the particular number.

You can control formatting more precisely by writing @dfn{modifiers}
between the @samp{%} and the character that indicates which conversion
to apply.  These slightly alter the ordinary behavior of the conversion.
For example, most conversion specifications permit you to specify a
minimum field width and a flag indicating whether you want the result
left- or right-justified within the field.

The specific flags and modifiers that are permitted and their
interpretation vary depending on the particular conversion.  They're all
described in more detail in the following sections.

@node Output Conversion for Matrices
@subsection Output Conversion for Matrices

When given a matrix value, Octave's formatted output functions cycle
through the format template until all the values in the matrix have been
printed.  For example:

@example
@group
printf ("%4.2f %10.2e %8.4g\n", hilb (3));

     @print{} 1.00   5.00e-01   0.3333
     @print{} 0.50   3.33e-01     0.25
     @print{} 0.33   2.50e-01      0.2
@end group
@end example

If more than one value is to be printed in a single call, the output
functions do not return to the beginning of the format template when
moving on from one value to the next.  This can lead to confusing output
if the number of elements in the matrices are not exact multiples of the
number of conversions in the format template.  For example:

@example
@group
printf ("%4.2f %10.2e %8.4g\n", [1, 2], [3, 4]);

     @print{} 1.00   2.00e+00        3
     @print{} 4.00
@end group
@end example

If this is not what you want, use a series of calls instead of just one.

@node Output Conversion Syntax
@subsection Output Conversion Syntax

This section provides details about the precise syntax of conversion
specifications that can appear in a @code{printf} template
string.

Characters in the template string that are not part of a
conversion specification are printed as-is to the output stream.

The conversion specifications in a @code{printf} template string have
the general form:

@example
% @var{flags} @var{width} @r{[} . @var{precision} @r{]} @var{type} @var{conversion}
@end example

For example, in the conversion specifier @samp{%-10.8ld}, the @samp{-}
is a flag, @samp{10} specifies the field width, the precision is
@samp{8}, the letter @samp{l} is a type modifier, and @samp{d} specifies
the conversion style.  (This particular type specifier says to print a
numeric argument in decimal notation, with a minimum of 8 digits
left-justified in a field at least 10 characters wide.)

In more detail, output conversion specifications consist of an
initial @samp{%} character followed in sequence by:

@itemize @bullet
@item
Zero or more @dfn{flag characters} that modify the normal behavior of
the conversion specification.
@cindex flag character (@code{printf})

@item
An optional decimal integer specifying the @dfn{minimum field width}.
If the normal conversion produces fewer characters than this, the field
is padded with spaces to the specified width.  This is a @emph{minimum}
value; if the normal conversion produces more characters than this, the
field is @emph{not} truncated.  Normally, the output is right-justified
within the field.
@cindex minimum field width (@code{printf})

You can also specify a field width of @samp{*}.  This means that the
next argument in the argument list (before the actual value to be
printed) is used as the field width.  The value is rounded to the
nearest integer.  If the value is negative, this means to set the
@samp{-} flag (see below) and to use the absolute value as the field
width.

@item
An optional @dfn{precision} to specify the number of digits to be
written for the numeric conversions.  If the precision is specified, it
consists of a period (@samp{.}) followed optionally by a decimal integer
(which defaults to zero if omitted).
@cindex precision (@code{printf})

You can also specify a precision of @samp{*}.  This means that the next
argument in the argument list (before the actual value to be printed) is
used as the precision.  The value must be an integer, and is ignored
if it is negative.

@item
An optional @dfn{type modifier character}.  This character is ignored by
Octave's @code{printf} function, but is recognized to provide
compatibility with the C language @code{printf}.

@item
A character that specifies the conversion to be applied.
@end itemize

The exact options that are permitted and how they are interpreted vary
between the different conversion specifiers.  See the descriptions of the
individual conversions for information about the particular options that
they use.

@node Table of Output Conversions
@subsection Table of Output Conversions
@cindex output conversions, for @code{printf}

Here is a table summarizing what all the different conversions do:

@table @asis
@item @samp{%d}, @samp{%i}
Print an integer as a signed decimal number.  @xref{Integer
Conversions}, for details.  @samp{%d} and @samp{%i} are synonymous for
output, but are different when used with @code{scanf} for input
(@pxref{Table of Input Conversions}).

@item @samp{%o}
Print an integer as an unsigned octal number.  @xref{Integer
Conversions}, for details.

@item @samp{%u}
Print an integer as an unsigned decimal number.  @xref{Integer
Conversions}, for details.

@item @samp{%x}, @samp{%X}
Print an integer as an unsigned hexadecimal number.  @samp{%x} uses
lowercase letters and @samp{%X} uses uppercase.  @xref{Integer
Conversions}, for details.

@item @samp{%f}
Print a floating-point number in normal (fixed-point) notation.
@xref{Floating-Point Conversions}, for details.

@item @samp{%e}, @samp{%E}
Print a floating-point number in exponential notation.  @samp{%e} uses
lowercase letters and @samp{%E} uses uppercase.  @xref{Floating-Point
Conversions}, for details.

@item @samp{%g}, @samp{%G}
Print a floating-point number in either normal (fixed-point) or
exponential notation, whichever is more appropriate for its magnitude.
@samp{%g} uses lowercase letters and @samp{%G} uses uppercase.
@xref{Floating-Point Conversions}, for details.

@item @samp{%c}
Print a single character.  @xref{Other Output Conversions}.

@item @samp{%s}
Print a string.  @xref{Other Output Conversions}.

@item @samp{%%}
Print a literal @samp{%} character.  @xref{Other Output Conversions}.
@end table

If the syntax of a conversion specification is invalid, unpredictable
things will happen, so don't do this.  In particular, @sc{matlab} allows
a bare percentage sign @samp{%} with no subsequent conversion character.
Octave will emit an error and stop if it sees such code.  When the string
variable to be processed cannot be guaranteed to be free of potential format
codes it is better to use the two argument form of any of the @code{printf}
functions and set the format string to @code{%s}.  Alternatively, for code
which is not required to be backwards-compatible with @sc{matlab} the
Octave function @code{puts} or @code{disp} can be used.

@example
@group
printf (strvar);        # Unsafe if strvar contains format codes
printf ("%s", strvar);  # Safe
puts (strvar);          # Safe
@end group
@end example

If there aren't enough function arguments provided to supply values for all
the conversion specifications in the template string, or if the arguments are
not of the correct types, the results are unpredictable.  If you supply more
arguments than conversion specifications, the extra argument values are
simply ignored; this is sometimes useful.

@node Integer Conversions
@subsection Integer Conversions

This section describes the options for the @samp{%d}, @samp{%i},
@samp{%o}, @samp{%u}, @samp{%x}, and @samp{%X} conversion
specifications.  These conversions print integers in various formats.

The @samp{%d} and @samp{%i} conversion specifications both print an
numeric argument as a signed decimal number; while @samp{%o},
@samp{%u}, and @samp{%x} print the argument as an unsigned octal,
decimal, or hexadecimal number (respectively).  The @samp{%X} conversion
specification is just like @samp{%x} except that it uses the characters
@samp{ABCDEF} as digits instead of @samp{abcdef}.

The following flags are meaningful:

@table @asis
@item @samp{-}
Left-justify the result in the field (instead of the normal
right-justification).

@item @samp{+}
For the signed @samp{%d} and @samp{%i} conversions, print a
plus sign if the value is positive.

@item @samp{ }
For the signed @samp{%d} and @samp{%i} conversions, if the result
doesn't start with a plus or minus sign, prefix it with a space
character instead.  Since the @samp{+} flag ensures that the result
includes a sign, this flag is ignored if you supply both of them.

@item @samp{#}
For the @samp{%o} conversion, this forces the leading digit to be
@samp{0}, as if by increasing the precision.  For @samp{%x} or
@samp{%X}, this prefixes a leading @samp{0x} or @samp{0X} (respectively)
to the result.  This doesn't do anything useful for the @samp{%d},
@samp{%i}, or @samp{%u} conversions.

@item @samp{0}
Pad the field with zeros instead of spaces.  The zeros are placed after
any indication of sign or base.  This flag is ignored if the @samp{-}
flag is also specified, or if a precision is specified.
@end table

If a precision is supplied, it specifies the minimum number of digits to
appear; leading zeros are produced if necessary.  If you don't specify a
precision, the number is printed with as many digits as it needs.  If
you convert a value of zero with an explicit precision of zero, then no
characters at all are produced.

@node Floating-Point Conversions
@subsection Floating-Point Conversions

This section discusses the conversion specifications for floating-point
numbers: the @samp{%f}, @samp{%e}, @samp{%E}, @samp{%g}, and @samp{%G}
conversions.

The @samp{%f} conversion prints its argument in fixed-point notation,
producing output of the form
@w{[@code{-}]@var{ddd}@code{.}@var{ddd}},
where the number of digits following the decimal point is controlled
by the precision you specify.

The @samp{%e} conversion prints its argument in exponential notation,
producing output of the form
@w{[@code{-}]@var{d}@code{.}@var{ddd}@code{e}[@code{+}|@code{-}]@var{dd}}.
Again, the number of digits following the decimal point is controlled by
the precision.  The exponent always contains at least two digits.  The
@samp{%E} conversion is similar but the exponent is marked with the letter
@samp{E} instead of @samp{e}.

The @samp{%g} and @samp{%G} conversions print the argument in the style
of @samp{%e} or @samp{%E} (respectively) if the exponent would be less
than -4 or greater than or equal to the precision; otherwise they use the
@samp{%f} style.  Trailing zeros are removed from the fractional portion
of the result and a decimal-point character appears only if it is
followed by a digit.

The following flags can be used to modify the behavior:
@c Not @samp so we can have ' ' as an item.

@table @asis
@item @samp{-}
Left-justify the result in the field.  Normally the result is
right-justified.

@item @samp{+}
Always include a plus or minus sign in the result.

@item @samp{ }
If the result doesn't start with a plus or minus sign, prefix it with a
space instead.  Since the @samp{+} flag ensures that the result includes
a sign, this flag is ignored if you supply both of them.

@item @samp{#}
Specifies that the result should always include a decimal point, even
if no digits follow it.  For the @samp{%g} and @samp{%G} conversions,
this also forces trailing zeros after the decimal point to be left
in place where they would otherwise be removed.

@item @samp{0}
Pad the field with zeros instead of spaces; the zeros are placed
after any sign.  This flag is ignored if the @samp{-} flag is also
specified.
@end table

The precision specifies how many digits follow the decimal-point
character for the @samp{%f}, @samp{%e}, and @samp{%E} conversions.  For
these conversions, the default precision is @code{6}.  If the precision
is explicitly @code{0}, this suppresses the decimal point character
entirely.  For the @samp{%g} and @samp{%G} conversions, the precision
specifies how many significant digits to print.  Significant digits are
the first digit before the decimal point, and all the digits after it.
If the precision is @code{0} or not specified for @samp{%g} or
@samp{%G}, it is treated like a value of @code{1}.  If the value being
printed cannot be expressed precisely in the specified number of digits,
the value is rounded to the nearest number that fits.

@node Other Output Conversions
@subsection Other Output Conversions

This section describes miscellaneous conversions for @code{printf}.

The @samp{%c} conversion prints a single character.  The @samp{-}
flag can be used to specify left-justification in the field, but no
other flags are defined, and no precision or type modifier can be given.
For example:

@example
printf ("%c%c%c%c%c", "h", "e", "l", "l", "o");
@end example

@noindent
prints @samp{hello}.

The @samp{%s} conversion prints a string.  The corresponding argument
must be a string.  A precision can be specified to indicate the maximum
number of characters to write; otherwise characters in the string up to
but not including the terminating null character are written to the
output stream.  The @samp{-} flag can be used to specify
left-justification in the field, but no other flags or type modifiers
are defined for this conversion.  For example:

@example
printf ("%3s%-6s", "no", "where");
@end example

@noindent
prints @samp{ nowhere } (note the leading and trailing spaces).

@node Formatted Input
@subsection Formatted Input

Octave provides the @code{scanf}, @code{fscanf}, and @code{sscanf}
functions to read formatted input.  There are two forms of each of these
functions.  One can be used to extract vectors of data from a file, and
the other is more `C-like'.

@DOCSTRING(fscanf)

@DOCSTRING(scanf)

@DOCSTRING(sscanf)

Calls to @code{scanf} are superficially similar to calls to
@code{printf} in that arbitrary arguments are read under the control of
a template string.  While the syntax of the conversion specifications in
the template is very similar to that for @code{printf}, the
interpretation of the template is oriented more towards free-format
input and simple pattern matching, rather than fixed-field formatting.
For example, most @code{scanf} conversions skip over any amount of
``white space'' (including spaces, tabs, and newlines) in the input
file, and there is no concept of precision for the numeric input
conversions as there is for the corresponding output conversions.
Ordinarily, non-whitespace characters in the template are expected to
match characters in the input stream exactly.  For example, note that
@code{sscanf} parses the string and whitespace differently when using
mixed numeric and string output types:
@cindex conversion specifications (@code{scanf})

@example
@group
teststr = "1 is a lonely number";
sscanf (teststr, "%s is a %s")
@result{} 1lonelynumber

sscanf (teststr, "%g is a %s")
@result{}
     1
   108
   111
   110
   101
   108
   121

[a, b, c] = sscanf ("1 is a lonely number", "%g is a %s %s", "C")
@result{} a =  1
@result{} b = lonely
@result{} c = number
@end group
@end example

When a @dfn{matching failure} occurs, @code{scanf} returns immediately,
leaving the first non-matching character as the next character to be
read from the stream, and @code{scanf} returns all the items that were
successfully converted.
@cindex matching failure, in @code{scanf}

The formatted input functions are not used as frequently as the
formatted output functions.  Partly, this is because it takes some care
to use them properly.  Another reason is that it is difficult to recover
from a matching error.

The specific flags and modifiers that are permitted in the template string
and their interpretation are all described in more detail in the following
sections.

@node Input Conversion Syntax
@subsection Input Conversion Syntax

A @code{scanf} template string is a string that contains ordinary
multibyte characters interspersed with conversion specifications that
start with @samp{%}.

Any whitespace character in the template causes any number of whitespace
characters in the input stream to be read and discarded.  The whitespace
characters that are matched need not be exactly the same whitespace
characters that appear in the template string.  For example, write
@samp{ , } in the template to recognize a comma with optional whitespace
before and after.

Other characters in the template string that are not part of conversion
specifications must match characters in the input stream exactly; if
this is not the case, a matching failure occurs.

The conversion specifications in a @code{scanf} template string
have the general form:

@example
% @var{flags} @var{width} @var{type} @var{conversion}
@end example

In more detail, an input conversion specification consists of an initial
@samp{%} character followed in sequence by:

@itemize @bullet
@item
An optional @dfn{flag character} @samp{*}, which says to ignore the text
read for this specification.  When @code{scanf} finds a conversion
specification that uses this flag, it reads input as directed by the
rest of the conversion specification, but it discards this input, does
not return any value, and does not increment the count of
successful assignments.
@cindex flag character (@code{scanf})

@item
An optional decimal integer that specifies the @dfn{maximum field
width}.  Reading of characters from the input stream stops either when
this maximum is reached or when a non-matching character is found,
whichever happens first.  Most conversions discard initial whitespace
characters, and these discarded characters don't count towards the
maximum field width.  Conversions that do not discard initial whitespace
are explicitly documented.
@cindex maximum field width (@code{scanf})

@item
An optional type modifier character.  This character is ignored by
Octave's @code{scanf} function, but is recognized to provide
compatibility with the C language @code{scanf}.

@item
A character that specifies the conversion to be applied.
@end itemize

The exact options that are permitted and how they are interpreted vary
between the different conversion specifiers.  See the descriptions of the
individual conversions in @ref{Table of Input Conversions} for
information about the particular options that they allow.

@node Table of Input Conversions
@subsection Table of Input Conversions
@cindex input conversions, for @code{scanf}

Here is a table that summarizes the various conversion specifications:

@table @asis
@item @samp{%d}
Matches an optionally signed integer written in decimal.  @xref{Numeric
Input Conversions}.

@item @samp{%i}
Matches an optionally signed integer in any of the formats that the C
language defines for specifying an integer constant.  @xref{Numeric
Input Conversions}.

@item @samp{%o}
Matches an unsigned integer written in octal radix.
@xref{Numeric Input Conversions}.

@item @samp{%u}
Matches an unsigned integer written in decimal radix.
@xref{Numeric Input Conversions}.

@item @samp{%x}, @samp{%X}
Matches an unsigned integer written in hexadecimal radix.
@xref{Numeric Input Conversions}.

@item @samp{%e}, @samp{%f}, @samp{%g}, @samp{%E}, @samp{%G}
Matches an optionally signed floating-point number.  @xref{Numeric Input
Conversions}.

@item @samp{%s}
Matches a string containing only non-whitespace characters.
@xref{String Input Conversions}.

@item @samp{%c}
Matches a string of one or more characters; the number of characters
read is controlled by the maximum field width given for the conversion.
@xref{String Input Conversions}.

@item @samp{%%}
This matches a literal @samp{%} character in the input stream.  No
corresponding argument is used.
@end table

If the syntax of a conversion specification is invalid, the behavior is
undefined.  If there aren't enough function arguments provided to supply
addresses for all the conversion specifications in the template strings
that perform assignments, or if the arguments are not of the correct
types, the behavior is also undefined.  On the other hand, extra
arguments are simply ignored.

@node Numeric Input Conversions
@subsection Numeric Input Conversions

This section describes the @code{scanf} conversions for reading numeric
values.

The @samp{%d} conversion matches an optionally signed integer in decimal
radix.

The @samp{%i} conversion matches an optionally signed integer in any of
the formats that the C language defines for specifying an integer
constant.

For example, any of the strings @samp{10}, @samp{0xa}, or @samp{012}
could be read in as integers under the @samp{%i} conversion.  Each of
these specifies a number with decimal value @code{10}.

The @samp{%o}, @samp{%u}, and @samp{%x} conversions match unsigned
integers in octal, decimal, and hexadecimal radices, respectively.

The @samp{%X} conversion is identical to the @samp{%x} conversion.  They
both permit either uppercase or lowercase letters to be used as digits.

By default, integers are read as 32-bit quantities.  With the @samp{h}
modifier, 16-bit integers are used, and with the @samp{l} modifier,
64-bit integers are used.

The @samp{%e}, @samp{%f}, @samp{%g}, @samp{%E}, and @samp{%G} conversions
match optionally signed floating-point numbers.  All five conversion
specifications behave identically, and will read in numerical values of
any floating point display style.

@node String Input Conversions
@subsection String Input Conversions

This section describes the @code{scanf} input conversions for reading
string and character values: @samp{%s} and @samp{%c}.

The @samp{%c} conversion is the simplest: it matches a fixed number of
characters, always.  The maximum field with says how many characters to
read; if you don't specify the maximum, the default is 1.  This
conversion does not skip over initial whitespace characters.  It reads
precisely the next @var{n} characters, and fails if it cannot get that
many.

The @samp{%s} conversion matches a string of non-whitespace characters.
It skips and discards initial whitespace, but stops when it encounters
more whitespace after having read something.

For example, reading the input:

@example
 hello, world
@end example

@noindent
with the conversion @samp{%10c} produces @qcode{" hello, wo"}, but
reading the same input with the conversion @samp{%10s} produces
@qcode{"hello,"}.

@node Binary I/O
@subsection Binary I/O

Octave can read and write binary data using the functions @code{fread}
and @code{fwrite}, which are patterned after the standard C functions
with the same names.  They are able to automatically swap the byte order
of integer data and convert among the supported floating point formats
as the data are read.

@DOCSTRING(fread)

@DOCSTRING(fwrite)

@node Temporary Files
@subsection Temporary Files

Sometimes one needs to write data to a file that is only temporary.
This is most commonly used when an external program launched from
within Octave needs to access data.  When Octave exits all temporary
files will be deleted, so this step need not be executed manually.

@DOCSTRING(mkstemp)

@DOCSTRING(tmpfile)

@DOCSTRING(tempname)

@DOCSTRING(tempdir)

@DOCSTRING(P_tmpdir)

@node EOF and Errors
@subsection End of File and Errors

Once a file has been opened its status can be acquired.  As an example
the @code{feof} functions determines if the end of the file has been
reached.  This can be very useful when reading small parts of a file
at a time.  The following example shows how to read one line at a time
from a file until the end has been reached.

@example
@group
filename = "myfile.txt";
fid = fopen (filename, "r");
while (! feof (fid) )
  text_line = fgetl (fid);
endwhile
fclose (fid);
@end group
@end example

@noindent
Note that in some situations it is more efficient to read the entire
contents of a file and then process it, than it is to read it line by
line.  This has the potential advantage of removing the loop in the
above code.

@DOCSTRING(feof)

@DOCSTRING(ferror)

@DOCSTRING(fclear)

@DOCSTRING(freport)

@node File Positioning
@subsection File Positioning

Three functions are available for setting and determining the position of
the file pointer for a given file.

@DOCSTRING(ftell)

@DOCSTRING(fseek)

@DOCSTRING(SEEK_SET)

@DOCSTRING(SEEK_CUR)

@DOCSTRING(SEEK_END)

@DOCSTRING(frewind)

The following example stores the current file position in the variable
@code{marker}, moves the pointer to the beginning of the file, reads
four characters, and then returns to the original position.

@example
@group
marker = ftell (myfile);
frewind (myfile);
fourch = fgets (myfile, 4);
fseek (myfile, marker, SEEK_SET);
@end group
@end example
